A Transport Block (TB) is data from an upper layer, e.g., from the Media Access Controller (MAC), that is given to the physical layer for transmission in a Transmission Time Interval (TTI). A TB may range in size from 16 bits to 36696 bits for a 10 Megahertz (MHz) Long Term Evolution (LTE) system. A TB may be split into Code Blocks (CBs), which have a maximum size of 6144 bits, meaning that the largest TB will have 7 code blocks. A TB Cyclic Redundancy Check (CRC) value is attached to the transport block. If a TB is divided into CBs, a CB CRC is attached to each CB.
In many wireless communications systems Hybrid Automatic Repeat Request (HARQ) retransmission is a method to mitigate unpredictable interference and channel variations. For downlink, when a wireless device attempts to decode a data message, the wireless device sends to the transmitter an indicator that indicates whether the decoding was successful or not. When the transmitter receives an indicator indicating unsuccessful decoding, the transmitter typically performs a retransmission of the data message, which the receiver typically will combine with the original received transmission. The combining is known as soft combining. Two well-known techniques for soft combining are “Chase combining,” in which every retransmission contains the same information, and “incremental redundancy,” in which every retransmission contains different information from the previous transmission, e.g., the retransmission contains only those portions of the original transmission that were not received correctly. The combining will greatly increase the probability of successful decoding.
In LTE the indicator indicating the result of a decoding attempt is known as a HARQ Acknowledgement (HARQ-ACK). For LTE up to two transport blocks (two data messages) may be transmitted in each Transmission Time Interval (TTI), which means that the HARQ-ACK may consist of 2 bits, where each bit indicates successful or unsuccessful receipt of the respective transport block.
LTE is a standard in the Third Generation Partnership Project (3GPP) family of wireless systems and is highly optimized for Mobile Broadband (MBB) traffic. The TTI in LTE is a subframe of 1 ms duration and the HARQ-ACK is, for Frequency Division Duplex (FDD), transmitted in subframe n+4 for a data transmission in subframe n. Ultra-Reliable Low Latency Communication (URLLC) is data service with extremely strict error and latency requirements. Error probabilities as low as 10−5 or lower and 1 ms end-to-end latency or lower are expected requirements.
Fifth Generation (5G) is currently being studied by 3GPP and is targeting a wide range of data services including MBB and URLLC. To enable optimized service, the length of TTI is expected to be different for different services, where URLLC may have a shorter TTI length compared to MBB.
It may then happen that, while a transmitter is in the middle of a MBB transmission, that transmitter may receive a URLLC data packet that is also to be transmitted. In this scenario, it may therefore be desirable for the transmitter to blank, i.e. interrupt, the MBB transmission in certain time-frequency resources and instead perform a URLLC transmission on those resources so that the URLLC transmission meets its latency and reliability requirements. This has the drawback that the wireless device receiving the partial or punctured MBB will likely fail the decoding, a situation which is handled by the HARQ feedback mechanism of retransmitting transport blocks that were not successfully received and decoded.